The purpose of this project is a detailed investigation at the molecular level of the mechanism of action of vitamin D and its most biologically active metabolite, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). The major thesis of the study is that in terms of its chemical structure and postulated mode of action the seco-steroid vitamin D is similar to that of other classical steroid hormones. In this regard the kidney is postulated to be the endocrine gland which produces in a physiologically regulated fashion small amounts of 1,25(OH)2D3. After systemic transport, the steroid interacts in its target tissues, the intestinal mucosa and bone to generate the usual biological responses attributed to vitamin D. The specific studies for the forthcoming year include: (a) development of a specific intestinal cytoplasmic receptor assay for 1,25(OH)2D3 and many related analogs such as 25(OH)D3, 24,25(OH)2D3, 1-hydroxy-vitamin D3, 3-deoxy 1,25-dihydroxyvitamin D; (b) isolation and purification of the "messenger RNA" for calcium binding protein (CaBP); (c) an evaluation of the mode of action of the "anti-vitamin" 19-hydroxy-dihydrovitamin D3-II; and (d) evaluation of the rates of biosynthesis and biodegradation of CaBP via incubation of intestinal mucosal cells with tritiated amino acids under in vitro conditions. BIBLIOGRAPHIC REFERENCES: Norman, A.W., J.D. Bayless and H.C. Tsai. Biologic effects of short term phenobarbital treatment on the response to vitamin D and its metabolites in the chick. Biochem. Pharmacol. 25, 163-168 (1976). Henry, H.L. and A.W. Norman. Studies on calciferol metabolism. XIII. Regulation of 25-hydroxy-vitamin D3-1-hydroxylase in isolated renal mitochondria. Arch. Biochem. Biophys. 172, 582-589 (1976).